2010-04-28

Spirit Sol 469

It's been nearly two weeks since I was last on shift. The drive we planned last time I was on shift went splendidly, putting us just where we wanted. Once there, as we started climbing up the long way around, the scientists started finding more stuff they just had to take a look at. So Spirit spent a while investigating some crumbly stuff called Methuselah, and now we've just driven up to a rock called Keystone, which they're practically salivating over. (And I'm not the only one who thinks so: "Steve's like a kid in a candy store," Dan Moyers remarks to me during the SOWG meeting.)

Thisol we're starting to IDD Keystone. Just to make sure I don't get bored, this is going to be the biggest MI mosaic of a single object ever, a total of 6x4x5+stereo = 144 images total. Our approach is to sequence as many of them today as the rover will have time to execute, and finish the rest tomorrow.

Before we can even really get started, we have to figure out what we want to image. This is troublesome, because we first have to know the angle of the MI with respect to the roughly rectangular rock surface -- that affects how the image footprints are oriented, which affects how you lay out the mosaic -- and none of our tools does that very well. So we basically just eyeball it, and it turns out it's reasonably close to the same orientation as the overall rectangle of rock itself, which simplifies things.

For the next step in the process, Ashitey and I (mostly Ashitey) pick a candidate center for the mosaic and use an image processing program to sketch out what the mosaic will cover with that as its center. Drawing the rectangle is easy, but figuring out where it lies is hard. No tools show you what the MI image covers (RSVP shows you a predicted MI image, but it's hard to figure out exactly where this lies in the original images), so you have to do a lot of eyeballing and detective work, and fit the results back into the original image. Oy. We email the resulting sketch to Steve, who prints it out, plays around with a ruler, and asks us to shift the whole thing 4.5cm to the left. So we do.

I think I have some ideas for improvements for future missions.

Anyway, with that done, the rest of the work isn't so hard. There are a lot of images in the mosaic, but they fall into groups, a total of 24 stacks of six images (including the stereo image) apiece. I further break down the mosaic into six columns of four stacks each, and then there's only a little overhead work to set up each column. It takes time, but I've had harder sequences to build. In the end, it turns out the rover won't have time to do more than four of the columns thisol, but that's still two-thirds of the total; we'll fill in the rest nextersol.

The only serious issue that arises is a question of using the IMU during the IDD sequence. This is something we do when there's a chance the vehicle's overall attitude would change during the sequence, which is something that could potentially endanger the IDD: if the vehicle tips forward when the arm's extended, suddenly it's that much closer to the ground, and could run into something we thought it would be clear of.

We're supposed to need to turn the IMU on when IDDing only when the rover's preloading the RAT or when we're at a significant pitch -- or anyway, that's what I thought the rules were. Ashitey thinks we need to do it when we're tilted, not just when we're pitched. (That is, he thinks we should do it even when we have zero pitch but lots of roll.) Thisol's pitch is only about 6.5 degrees but the overall tilt is a much higher 14.5 degrees, so it's going to matter which of us is right. John Wright is around, so I bring him in as a tie-breaker, and he votes with Ashitey, darn it. ("That was the wrong answer," I tell him.[1])

Why not just turn on the IMU anyway? Spirit's got power to burn, so it's not that. The problem is that there's ten seconds of overhead in every IDD move when the IMU is on, and what with all the crazy stuff we're doing thisol, the ten seconds per move add up to a whopping 38 minutes of overhead! And we don't have that to spare.

We could make back the 38 minutes by cutting another column, but John comes up with a better idea than that. It's a compromise: turn on the IMU during the unstow and first placement, and thereafter only when we actually contact the surface. Those are the times when there's the greatest risk (not that there's any real risk, but we're paid to be paranoid and we earn every penny) that the attitude would change. And that compromise adds only about six or eight minutes to the run-time, not 38. So that's what we do.

It's another fairly long day, but still less than ten hours. And if these images look as cool as I hope they will, it will have been well worth it.